I had this problem some time ago and John provided me the solution.. but
I cannot remember, sorry.
I've already run autogen.sh but maybe I forgot some other script...
Here's the error:
fede@debian:~/lilypond-git/Documentation$ make ISOLANG=IT
check-translation
ISOLANG=IT /usr/bin/python ../scripts/auxiliar/check_translation.py
Traceback (most recent call last):
File "../scripts/auxiliar/check_translation.py", line 24, in<module>
import langdefs
ImportError: No module named langdefs
make: *** [check-translation] Error 1